Structure and function analysis of the calcium-related gene spray in Neurospora crassa.

The spray gene was cloned, and wildtype and mutant alleles were sequenced. Spray(+) has a 3452-bp open reading frame plus seven introns. The spray mutant had a T --> G transversion close to the carboxyl end, creating a stop codon (TGA). The sequence shows no match to genes of known function, but the carboxyl end shows seven transmembrane domains and matches putative membrane proteins of yeast. The most abundant transcript detected was 4.4 kb in size. Repeat-induced point mutagenesis produced the mutant spray phenotype. Electrophysiological analysis showed that ion fluxes in the spray plasma membrane are normal; furthermore, whereas the spray mutant was known to have no organelle-based calcium fluorescence, the cytosol shows a tip-high calcium gradient. The spray mutant is sensitive to calcineurin inhibitors. The results suggest that the SPRAY protein is located in an organellar membrane, regulating the distribution of Ca(2+) via calcineurin.

Tests of a cellular model for constant branch distribution in the filamentous fungus Neurospora crassa.

The growth of mycelial fungi is characterized by the highly polarized extension of hyphal tips and the formation of subapical branches, which themselves extend as new tips. In Neurospora crassa, tip growth and branching are crucial elements for this saprophyte in the colonization and utilization of organic substrates. Much research has focused on the mechanism of tip extension, but a cellular model that fully explains the known phenomenology of branching by N. crassa has not been proposed. We described and tested a model in which the formation of a lateral branch in N. crassa was determined by the accumulation of tip-growth vesicles caused by the excess of the rate of supply over the rate of deposition at the apex. If both rates are proportional to metabolic rate, then the model explains the known lack of dependence of branch interval on growth rate. We tested the model by manipulating the tip extension rate, first by shifting temperature in both the wild type and hyperbranching (colonial) mutants and also by observing the behavior of both tipless colonies and colonyless tips. We found that temperature shifts in either direction result in temporary changes in branching. We found that colonyless tips also pass through a temporary transition phase of branching. The tipless colonies produced a cluster of new tips near the point of damage. We also found that branching in colonial mutants is dependent on growth rate. The results of these tests are consistent with a model of branching in which branch initiation is controlled by the dynamics of tip growth while being independent of the actual rate of this growth.

Kalilo plasmids are a family of four distinct members with individual global distributions across species.

Kalilo is a linear 9-kb plasmid, isolated originally from Hawaiian strains of the heterothallic fungus Neurospora intermedia. Its properties include terminal inverted repeats, two ORFs coding for a presumptive DNA and an RNA polymerase, and the ability to cause senescence in its original host and in the closely related species Neurospora crassa. We have examined natural isolates alleged to contain plasmids homologous to kalilo. Most of these isolates do in fact contain plasmids with so close an identity to kalilo as to be certain relatives. We found a new case of kalilo in Neurospora tetrasperma from Moorea-Tahiti, and a new case of LA-kalilo (previously found only in N. tetrasperma) in N. crassa from Haiti. A previously unreported, substantially shorter, kalilo variant has been found in three geographically separate isolates of the heterothallic species Neurospora discreta. Therefore, if the previously reported kalilo variant from the genus Gelasinospora is included, in all there are four members of the kalilo plasmid family. The main differences between these plasmids are in the terminal inverted repeats (TIRs). The phylogeny of the TIR sequences is largely congruent with that of nuclear DNA in the species in which they are found, suggesting that the plasmids are related by vertical descent throughout the evolution of these species. However, there are two cases of a plasmid found in a heterothallic and a pseudohomothallic species in the same global area; these cases might have arisen from more recent horizontal transmission or introgression.

Possible benefits of kalilo plasmids to their Neurospora hosts.

Neurospora mitochondrial plasmids are ubiquitous in natural populations, yet many of them are lethal to their host strains or seem to impose a molecular genetic load. Five pairs of strains of Neurospora tetrasperma and N. crassa with and without kalilo-like plasmids were tested under a variety of situations. The purpose was to find possible beneficial effects of plasmids that might offset their disadvantages. We found that, in all cases tested, plasmids conferred an advantage to growth at temperatures close to the top of the range for this fungus. Also, the plasmids improved fertility, as measured by perithecial production. Negative results were obtained for heavy metal resistance and ascospore germination. The results generate the hypothesis that plasmids may have adaptive significance to their hosts.

Transfer of Neurospora kalilo plasmids among species and strains by introgression.

There are four different variants of the kalilo "family" of linear mitochondrial plasmids. This family is found in several heterothallic species and one pseudohomothallic species of Neurospora, as well as in one homothallic species of Gelasinospora. The mode of dispersal of these plasmids is not known. Horizontal transmission has proved difficult to demonstrate. Another possibility is transfer by introgression, and this is modelled in the present paper. We have used introgression and subsequent heterokaryosis to successfully transfer the LA-kalilo plasmid from a Haitian strain of Neurospora crassa to the standard Oak Ridge N. crassa background, the LA-kalilo plasmid from the pseudohomothallic Neurospora tetrasperma to N. crassa, and the kalilo plasmid from N. crassa to N. tetrasperma. Thus, introgression is shown to be a possible avenue of dispersal between species. The recipient strains were all senescent but the mechanism of this senescence is not known. It could be caused by the plasmids, but if so the mechanism is novel since plasmid/mtDNA junction fragments of the type found in the standard mode of mtDNA insertion could not be detected. However, mtDNA changes were observed in the senescent recipients.

The frost gene of Neurospora crassa is a homolog of yeast cdc1 and affects hyphal branching via manganese homeostasis.

The Neurospora crassa mutant frost has a hyperbranching phenotype that can be corrected by adding Ca(2+), suggesting that characterization of this gene might clarify the mechanism of Ca(2+)-dependent tip growth. The wild-type allele was cloned by sib selection using protoplasts from arthroconidia. RFLP analysis revealed that the cloned DNA fragment mapped to the fr locus. The nucleotide sequence of genomic and cDNA was determined. The deduced amino acid sequence showed homology to the Saccharomyces cerevisiae CDC1 protein, implicated in manganese homeostasis. The fr mutant was sensitive to Mn(2+), and a revertant allele whose product differs by one amino acid was tolerant to Mn(2+). Mn(2+) depletion induced the wild-type strain to hyperbranch, resulting in a morphology similar to that of fr. The fr mutant was also sensitive to calcineurin inhibitors. These results suggest that fr is involved in Mn(2+) homeostasis and point to a role for Mn(2+) in Neurospora branching.

hcf5, a nuclear photosynthetic electron transport mutant of Arabidopsis thaliana with a pleiotropic effect on chloroplast gene expression.

A photosynthetic mutant of Arabidopsis thaliana, hcf5, was isolated by screening M2 seedlings for high chlorophyll fluorescence. Thylakoid morphology was strikingly abnormal, with large grana stacks and almost no stroma lamellae. Fluorescence induction kinetics, activity assays, and immunoblotting showed that photosystem II was absent. Polypeptides of the photosystem I complex, the Cyt b6/f complex, coupling factor, and the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase were also severely depleted. However, the nuclear-encoded chlorophyll a/b light-harvesting complex polypeptides were unaffected. The rbcL transcript was present at very low levels, the pattern of transcripts from the polycistronic psbB-psbH-petB-petD operon was abnormal, and the mature psbH message was almost completely lacking. This suggests that the hcf5 locus may encode a product required for the correct expression of several chloroplast genes.

The molecular nature of mutations in the mt A-1 gene of the Neurospora crassa A idiomorph and their relation to mating-type function.

The 293-amino acid mt A-1 ORF of the A mating-type idiomorph of Neurospora crassa is multifunctional. It confers A mating identity and is responsible for heterokaryon incompatibility. The goal of this study was to dissect the functional regions of mt A-1. New mutants of mt A-1 selected for loss of the incompatibility function were obtained. One new mutant, A(m)99, was partially fertile as a maternal parent. This is the first time that fertility and incompatibility functions have been separated for the A idiomorph. In this mutant, the mt A-1 ORF is truncated after the first 85 amino acids, indicating that this N-terminal region is minimally sufficient for female fertility. A series of deletion constructs and frameshift alleles of mt A-1 was obtained and tested for male-mating activity and vegetative incompatibility in transformation experiments. These experiments showed that a region from position 1 to 111 is sufficient to confer incompatibility, while amino acids from position 1 to 227 are required for mating activity. A transcriptional analysis of mt A-1 showed that the mRNA is expressed both before and after fertilization. This, together with the phenotype of the A(m)99 mutant, suggests a post-fertilization function for mt A-1.

Structure of a Gelasinospora linear plasmid closely related to the kalilo plasmid of Neurospora intermedia.

We have determined the complete nucleotide sequence of a linear mitochondrial plasmid from a natural isolate of a homothallic species of Gelasinospora. The plasmid genome is 8231 bp long. It carries terminal inverted repeats of 1137 bp. Extending inwards from the terminal repeats are two long open reading frames coding for putative proteins with similarity to DNA and RNA polymerases. These are separated by a short intergenic region. The plasmid sequence shows remarkable similarity to that of the Neurospora intermedia senescence-plasmid kalilo. Overall the two plasmids have a similar genetic organization and are clearly homologous at the sequence level. The main differences are in the intergenic region and in the terminal repeats.

Natural plasmids of filamentous fungi.

Among eukaryotes, plasmids have been found in fungi and plants but not in animals. Most plasmids are mitochondrial. In filamentous fungi, plasmids are commonly encountered in isolates from natural populations. Individual populations may show a predominance of one type, but some plasmids have a global distribution, often crossing species boundaries. Surveys have shown that strains can contain more than one type of plasmid and that different types appear to be distributed independently. In crosses, plasmids are generally inherited maternally. Horizontal transmission is by cell contact. Circular plasmids are common only in Neurospora spp., but linear plasmids have been found in many fungi. Circular plasmids have one open reading frame (ORF) coding for a DNA polymerase or a reverse transcriptase. Linear plasmids generally have two ORFs, coding for presumptive DNA and RNA polymerases with amino acid motifs showing homology to viral polymerases. Plasmids often attain a high copy number, in excess of that of mitochondrial DNA. Linear plasmids have a protein attached to their 5' end, and this is presumed to act as a replication primer. Most plasmids are neutral passengers, but several linear plasmids integrate into mitochondrial DNA, causing death of the host culture. Inferred amino acid sequences of linear plasmid ORFs have been used to plot phylogenetic trees, which show a fair concordance with conventional trees. The circular Neurospora plasmids have replication systems that seem to be evolutionary intermediates between the RNA and the DNA worlds.

The dynamics of mitochondrial plasmids in a Hawaiian population of Neurospora intermedia.

We analysed the distribution of mitochondrial plasmids among 82 Neurospora intermedia isolates from Hawaii; 74% of the isolates carried the neutral circular plasmid Han-2, whereas 38% contained the linear senescence-causing plasmid kalDNA. The distributions of the two plasmids are independent. There is no significant difference between the Kauaian population of 1972 and that of 1976. To further examine the reasons for this frequency distribution we studied the transmission of both Hawaiian plasmids through the maternal parent in a large series of crosses using non-Kalilo isolates as conidial parents. Plasmids can be lost during the sexual cycle. The Han-2 plasmid is transmitted more efficiently than kalDNA. No clear cases of autonomous or non-autonomous plasmid suppression were observed, so loss can be considered accidental. One Kalilo strain proved to be ineffectual as a maternal parent, and this reduced its ability to transmit kalDNA to the next generation. The dynamic balance of plasmids in natural populations over time is probably a result of the interplay of many forces, including those described in this work and those from several other studies on Neurospora plasmids.

Recombination between heterologous linear and circular mitochondrial plasmids in the fungus Neurospora.

A strain of Neurospora intermedia from China contains five prominent extragenomic mitochondrial plasmids: three linear elements called zhisi plasmids, and two circular plasmids, Harbin-1 and -2. In one subculture, levels of four plasmids (all three zhisis and Harbin-1) fell to undetectable values and two novel linear plasmids appeared, Harbin-L and -L2, as well as a new small circular plasmid, Harbin-0.9. Cross-hybridization of restriction fragments and DNA sequencing showed that the Harbin-L plasmid was composed of parts of the circular Harbin-1 plasmid and of one of the linear zhisi plasmids. A model is presented in which the Harbin-1 and zhisi plasmids are present within the same mitochondrion, and crossovers at two separate 7 bp sites of sequence identity effectively insert part of the circular Harbin-1 DNA into a zhisi linear plasmid, simultaneously deleting part of the zhisi element. The small plasmid Harbin-0.9 is a fragment of the Har-1 plasmid, and seems to be another product of the recombination process that created Har-L. Recombination of this type could have contributed to the wide array of mitochondrial plasmids found in natural populations of Neurospora.

A kalilo-like linear plasmid in Louisiana field isolates of the pseudohomothallic fungus Neurospora tetrasperma.

Two Louisiana strains of Neurospora tetrasperma contain a linear plasmid (LA-kalDNA) with a restriction map identical to the Hawaiian Neurospora intermedia senescence plasmid, kalDNA, but with termini 100 nucleotide pairs shorter. One of these strains also bore a circular plasmid similar to the Hawaiian circular plasmid Hanalei-2. One species probably acquired both plasmids from the other by horizontal transfer, at a time sufficiently distant for sequence divergence to take place. Many LA-kalDNA-bearing derivative strains senesced, but this plasmid does not guarantee senescence. Furthermore, LA-kalDNA does not insert into mtDNA. One senescent strain showed no LA-kalDNA. The plasmids are effectively transmitted via the pseudohomothallic sexual cycle. Single mating-type derivatives transmit plasmids maternally.

Vegetative incompatibility in Neurospora: its effect on horizontal transfer of mitochondrial plasmids and senescence in natural populations.

We have investigated the horizontal transfer of two mitochondrial plasmids and the Kalilo senescence phenotype in the fungus Neurospora without the use of heterokaryon-forcing markers. The Kalilo senescent state was only transferred between fully-compatible N. crassa strains, but not between strains differing at any of the loci het-c, het-d, het-e or mating-type. However, the linear plasmid kalDNA and the circular plasmid Han-2 were transferred following incompatible vegetative interactions. Our data suggest that vegetative incompatibility due to allelic differences at het-c is more effective in preventing transfer than that due to het-d, het-e or mating-type. Based on these observations we have developed a novel test for assessing vegetative incompatibility between Kalilo and non-Kalilo field isolates of N. intermedia. In this procedure combinations of Kalilo and non-Kalilo field isolates of N. intermedia were grown together and tested for senescence. Compatibility is inferred if the young non-Kalilo strain dies along with the senescent Kalilo strain, whereas incompatibility is inferred when the Kalilo strain dies without imposing its senescent state onto the non-Kalilo strain. Our results suggest that each of the nine Kalilo strains tested is incompatible with each of 20 non-Kalilo isolates from the same N. intermedia population of the Hawaiian island of Kauai. However, the observed incompatibility did not completely prevent cytoplasmic exchange, and in several cases plasmid transfer could be detected.

New mutations that suppress mating-type vegetative incompatibility in Neurospora crassa.

The mating-type locus in the ascomycete Neurospora crassa functions as a vegetative-incompatibility locus during asexual growth such that A+a heterokaryons and A/a partial diploids are inhibited in their growth. In this study, we sought mutations that suppress mating-type associated vegetative incompatibility by using A/a partial diploids. Mutants were selected as spontaneous escapes from inhibited growth. Suppressors were identified by selecting escapes that retained the capacity to mate with both A and a strains. The escaped partial diploids were then outcrossed to remove the duplication. Seven suppressors were identified that segregated as single, recessive mutations. All seven of the suppressors were shown to be allelic to a previously obtained suppressor, tol, by segregation analyses and heterokaryon tests. This result indicates that all seven mutations are new alleles of tol and suggests that tol is a key regulatory locus in the expression of mating-type associated vegetative incompatibility in N. crassa.

A nuclear photosynthetic electron transport mutant of Arabidopsis thaliana with altered expression of the chloroplast petA gene.

The nuclear photosynthetic mutant, hcf2, of Arabidopsis thaliana was isolated by screening M2 seedlings for abnormally-high chlorophyll fluorescence (hcf), indicative of a block in photosynthetic electron transport. Fluorescence induction kinetics, photosynthetic electron transport activity assays and immunoblotting revealed that all the complexes involved in photosynthetic electron transport were affected to some extent. The most striking effect of the mutation was on the relative steady state levels of the petA transcript (encoding the apoprotein of cytochrome f) which were more than five-times higher in the mutant plants than in their wild-type siblings.

Forearm substrate utilization during exercise after a meal containing both fat and carbohydrate.

1. Whilst the provision of exogenous carbohydrate has been shown to be beneficial to endurance exercise performance, little attention has been paid to the possibility of dietary manipulation of the availability of fat. 2. Ten normal subjects were studied on two occasions: after an overnight fast (postabsorptive state) and after a meal containing 80 g of fat and 80 g of carbohydrate (fed state). Forearm substrate exchange was studied during 60 min of isometric forearm exercise (5 s contraction, 5 s relaxation). 3. In the fed state concentrations of plasma triacylglycerol (1510 +/- 150 versus 850 +/- 80 mumol/l, P < 0.01), blood ketone bodies (151 +/- 21 versus 80 +/- 10 mumol/l, P < 0.01) and plasma insulin (17 +/- 3 versus 7 +/- 1 m-units/l, P < 0.01) were elevated compared with the postabsorptive state; plasma glucose and non-esterified fatty acid concentrations were not significantly different. 4. Forearm blood flow and O2 consumption each increased about 6-fold during exercise, with no differences between the two nutritional states. The potential contribution of individual substrates to forearm O2 consumption (a calculation which is independent of blood flow) was assessed: for triacylglycerol it was significantly greater in the fed state (P < 0.01). The sum of the potential contributions of triacylglycerol, non-esterified fatty acids, glucose and ketone bodies to forearm O2 consumption was significantly greater, both before and during exercise, in the fed than in the postabsorptive state (P < 0.05), implying the sparing of endogenous (forearm) fuels. 5. These studies highlight the potential for manipulation of substrate supply during exercise by feeding meals containing both carbohydrate and fat.

Immediate metabolic availability of dietary fat in combination with carbohydrate.

We tested the hypothesis that when fat is ingested in combination with carbohydrate, direct release of fatty acids into the plasma may occur. Eight normal subjects ingested two meals: high-fat (80 g fat, 80 g carbohydrate, and 18 g protein) and low-fat (< 1 g fat, otherwise closely matched). Over the subsequent 6 h, net fat oxidation was greater after the high-fat meal (20.7 vs 10.6 g, P < 0.01). Plasma nonesterified fatty acid (NEFA) concentrations were markedly suppressed after the low-fat meal, but relatively maintained after the high-fat meal (P < 0.01). The profile of plasma NEFAs changed after the high-fat meal, consistent with entry of meal-derived fatty acids into the NEFA pool. We suggest that after ingestion of combinations of carbohydrate and fat, the action of lipoprotein lipase on chylomicron-triacylglycerol leads to direct release of fatty acids into the plasma and increased fat oxidation.

Plasmid suppressors active in the sexual cycle of Neurospora intermedia.

We have discovered that, in certain crosses of natural isolates of Neurospora intermedia, linear and circular mitochondrial plasmids of the maternal parent are not transmitted to the progeny. This contrasts with the maternal transmission of organellar genetic elements generally observed in crosses between laboratory strains and between other natural isolates. Formally, failure of plasmid transmission is a type of plasmid suppression. The present cases represent the first report of plasmid suppressors in natural populations of fungi. Strains used as female parents can transmit or not transmit plasmids depending on the strain used as male parent. Males that act to suppress in one cross fail to suppress in others. Therefore, the suppression of plasmids depends on a strain-specific interaction and is not determined exclusively by the males. Since suppression is a specific interaction we inferred that it must be genetically based and tested this hypothesis by seeking segregation of suppressed and nonsuppressed phenotypes in octads. Segregation of the original full suppression of all plasmids was indeed observed in each of the three sets of testcrosses examined. The interaction type of suppression must be initiated in ascogenous tissue during the sexual cycle. It is a nonautonomous type of suppression, affecting all descendent cells. In any one case of suppression, either one, several, or all plasmids can be lost. Both linear and circular plasmids can be eliminated by the same suppressor genotype. In addition, several strains were found to contain suppressors that act after ascospore delineation. This autonomous type of suppression has been observed previously in laboratory strains, but not in natural isolates. All the cases of plasmid suppression identified in this study involved a range of apparently neutral circular and linear plasmids. Using one senescent Kalilo strain of N. intermedia, we did not detect any case of suppression of the senescence-determining linear plasmid kalDNA.